Such a diaphragm, annular in shape, is described for example in the specification of published French Patent Application FR2244101A and the corresponding United Kingdom Patent Specification No. 1438310. It comprises a peripheral portion which defines a Belleville ring, and a central portion which is divided into radial fingers by slots. In its relaxed state the general shape of the whole is frustoconical. The peripheral portion of the diaphragm constitutes an axially acting resilient means which, when installed in a clutch, is engaged on a cover plate of the clutch and bears on a pressure plate of the clutch, in such a way that the latter is continuously biased into engagement such that at least one friction wheel or clutch plate of the clutch is gripped. The radial fingers of the central portion of the diaphragm act as declutching levers which, under the action of a clutch release bearing, are adapted to cause the pressure plate to be released by the Belleville ring portion of the diaphragm, so that the pressure plate is then itself released.
In the vicinity of the root of the radial fingers, and alternating with the latter, the diaphragm is formed with apertures, each of which is open radially into a respective one of the slots by which the radial fingers are separated from each other. In the context of clutches of the kind known as push-off clutches, some of these apertures are used as passages through which lugs, spacer bars or the like pass. These lugs, spacer bars or the like form at least part of a pivoting connection between the diaphragm and the cover plate of the clutch.
One difficulty encountered in the design of the diaphragm results from the fact that in service, during a declutching operation, the radial fingers of its central portion are not bent through an inclination the same as that found in its peripheral portion. In practice, the bending inclination of the radial fingers is greater than that in the peripheral portion. As a result, for a given displacement of the clutch release bearing, the clutch is released by an amount which is less than the amount by which the pressure plate is released. In addition, there is a possibility of undesirable wrinkling occurring in the diaphragm, and more precisely in its Belleville ring portion, such as to cause fracture of the diaphragm.
In addition, as is described in the specification of French published Patent Application FR2563877A, a diaphragm has a characteristic curve having a peaked shape, in which the diaphragm force increases firstly as the deformation of the diaphragm increases, and then passes through a maximum before decreasing again. The elastic characteristic of the diaphragm is determined by the ratio h/s (where h is the height of the frustum of a cone defined by the Belleville ring, measured parallel to the axis, and s is the thickness of the diaphragm).
For this reason, in the last mentioned document, an arrangement is provided in which the Belleville ring is formed with an inward curvature so as to reduce the peaking effect of the characteristic curve of the diaphragm. However, such an arrangement is not entirely satisfactory, because firstly, it does not lead to good control of the force exerted by the diaphragm, and secondly, it stiffens the peripheral portion of the diaphragm in such a way that the bending of the fingers is even more pronounced.